sessiond: return raw pointer to packet header

There is no benefit to transfering the ownership of a new packet header
everytime it is serialized. Packet headers don't change after the
creation of a session; prefer allocating it once and provide a
raw (const) pointer to the session's instance.

While LTTng always produces one, a trace doesn't absolutely need a
packet header so a pointer is preferred over a reference.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I66b8f9ad74e332edcc8273c076137a7fff0d4d5e